1. Field of the Invention
The present invention relates to a plane illumination apparatus and a backlight apparatus that use a light source for emitting coherent light beams.
2. Description of Related Art
As a backlight apparatus used in liquid crystal panels or the like, a system in which light is incident on an edge of a light guide plate, repeatedly reflected between two opposing surfaces by total reflection, and then taken out by a diffusion device or the like is known. This type of backlight apparatus includes backlight apparatuses using a cold cathode fluorescent lamp as a light source and also, recently, backlight apparatuses using an LED as a light source.
When a cold cathode fluorescent lamp is used, there is a problem in that it is difficult to make a backlight apparatus thin and power consumption increases. When an LED is used, although it is possible to make a backlight apparatus thin, since an LED is a uniform diffusion illumination device, it is difficult to make every light incident on a thin light guide plate with no leakage, hence loss is caused.
In contrast to above, a laser beam is excellent in straightness, and hence considered to improve light incidence efficiency.
However, when a laser beam is used as a light source, speckles due to high coherency of laser are generated. Speckles are a spotted pattern which is formed when a coherent light beam such as a laser beam is emitted to a scattering plane. If speckles are generated on a screen, they are observed as spotted luminance unevenness, i.e. brightness unevenness, thus becoming a factor of having physiologically adverse affect on an observer. It is considered that the reason why speckles are generated in the case of using coherent light beams is that coherent light beams reflected from respective portions of a scattering and reflecting plane such as a screen have very high coherency so that coherent light beams interfere with one another to generate speckles. For example, a theoretical review of the generation of speckles is made in detail in Speckle Phenomena in Optics, Joseph W. Goodman, Roberts & Co., 2006.
As discussed above, in the system using a coherent light source, since there is a problem of generation of speckles unique to the coherent light source, techniques for suppressing the generation of speckles have been proposed. For example, Japanese Patent Laid-Open No. 6-208089 discloses a technique in which a laser beam is emitted to a scattering plate, scattered light beams obtained therefrom are guided to a spatial light modulator, and the scattering plate is driven to rotate by a motor, thus reducing speckles.